Oxide films form on aluminum alloys when they are exposed to an atmosphere containing oxygen. Specifically, aluminum readily oxidizes in the presence of air (Eqn. (1)), or moisture (Eqns. (2 and 3)), rapidly forming a thin, strong protective oxide film on any exposed aluminum metal surface, including both liquid and solid surfaces. Aluminum, particularly at elevated temperature has very high inherent potential for oxidation (Al2O3) when its surfaces are exposed to oxygen-containing environment.4Al+3O2→2Al2O3  (1)3H2O+2Al→Al2O3+3H2  (2)H2→2[H]melt  (3)
Because aluminum oxide is very stable thermodynamically, it is typically present in all aluminum alloys. Therefore, any furnace charge contains unavoidable amounts of alumina as a typical coating. During the mold filling of the casting process, additional aluminum oxides are formed when the free surface of the melt front contacts air and particularly when the liquid melt velocity produces turbulent flow. A distinction is often made between oxides pre-existing in the melting furnace, referred to as “old oxides,” and those created during mold filling, called “young oxides.” Campbell, J., Castings, Elsevier Butterworth-Heinemann, 2003; Q. G. Wang, C. J. Davidson, J. R. Griffiths, and P. N. Crepeau, “Oxide Films, Pores and The Fatigue Lives of Cast Aluminum Alloys”, Metall. Mater. Trans. vol. 37B (2006), pp. 887-895.
The “old” oxides formed before and during melting can be suspended in the melt and be transferred to the casting. The oxides cause a variety of downstream problems unless special precautions are taken. For example, entrained oxides are believed to increase melt viscosity and apparent surface tension, hence reducing fluidity and adversely affecting feeding of castings.
The presence of oxides in aluminum alloys has also been well recognized to be detrimental to the mechanical properties, in particular fatigue performance, of aluminum castings. Oxide films have been reported as the second most common origin for fatigue crack initiation in aluminum castings. In addition to a reduction in mechanical properties, oxidation results in the consumption of valuable aluminum.
In melting and re-melting of aluminum alloys, a significant amount of aluminum dross can be produced on the top of the molten aluminum metal in the furnace. During the melting process, an average of 4-5% of the input material to the furnace is lost to oxidation. When the aluminum dross is skimmed off the liquid melt surface, up to 90% of the material removed is free aluminum metal. Dross, therefore, in an aluminum melting operation, includes a significant amount of aluminum metal.
The dross skimmed off from the liquid melt surface in the furnace is usually slowly cooled down on a foundry floor surface. Free aluminum metal in the dross is further lost due to a thermite reaction, i.e., exothermic oxidation of aluminum metal with a huge fresh melt surface. Typically, about 2% of the aluminum metal in the dross is lost for each minute of cooling. Therefore, a dross initially having about 80% aluminum metal will decrease to about 40% to 60% aluminum metal after cooling because of losses due to the thermite reaction.
To minimize and eventually eliminate the oxides in the final cast aluminum products, it is desired to reduce old oxides in the liquid metal to be fed to a mold for casting.
Although there is a strong practical need to remove old oxides from the liquid metal melt, no reliable method or technique has yet been reported.